The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
When an engine is initially started, a catalyst in an exhaust system of the engine may be heated to a light-off temperature to maximize the ability of the catalyst to reduce emissions. The period during which the catalyst is heated may be referred to as catalyst light-off. During catalyst light-off, an engine may be controlled to produce exhaust having a lean air/fuel ratio to heat the catalyst to the light-off temperature.
During normal operation, the engine may be controlled to maintain an air/fuel ratio of the engine at a stoichiometric ratio. The air/fuel ratio of the engine may be controlled based on output from an oxygen sensor positioned upstream from the catalyst. However, when the engine is initially started, the output from the oxygen sensor may not be reliable until the oxygen sensor is heated to an activation temperature. Thus, the air/fuel ratio of the engine may be controlled based on other engine operating conditions, such as intake airflow, when the engine is initially started.